Multi-resolution optical spectrometer
The invention relates to a multi-resolution optical spectrometer that employs two output lenses of different focal length to provide a broad wavelength range, coarse resolution spectral measurement and a high resolution, lower range spectral measurement. Light dispersed by a virtual image phase array followed by a diffraction grating in two different dispersion orders may be separately focused by the two lenses upon to 2D detector array to provide the two measurements.
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The present invention claims priority from U.S. Provisional Patent Application No. 62/447,107, which is entitled “VIPA spectrometer having multiple spectral resolution ranges”, filed Jan. 17, 2017, which is incorporated herein by reference.
TECHNICAL FIELDThe present invention generally relates to optical spectrometers, and more particularly to multi-resolution optical spectrometers that disperse light in two dimensions.
BACKGROUND OF THE INVENTIONHigh resolution optical spectrometers typically require highly dispersive optical elements. One example of such optical element is a virtually imaged phase array (VIPA), which is an optical component that has a very high angular dispersion D=dθ/dλ, where θ is a dispersion angle at which light of wavelength λ is dispersed by the VIPA. A conventional VIPA includes two parallel surfaces, one of which typically has a highly-reflective coating, and the other is partially-reflective. The highly-reflective surface has an input zone 6 that may have an anti-reflection coating through which input light may enter the VIPA at an angle, to be reflected back and forth across a gap between the highly and partially reflective surfaces, gradually leaking out through the partially reflective surface. Because the two reflective surfaces are highly parallel, the output beams have a well-defined phase relationship, which interference in a focal plane of a lens results in a strong angular dispersion that enables the use of the VIPA in a spectrometer.
One limitation of a VIPA is a relatively small free spectral range (FSR), which is typically only about 50-80 times greater than the spectral resolution of the VIPA. If two wavelengths incident on the VIPA are separated by one FSR, their angular locations at the detector will be identical. In order to broaden the overall operating range of the spectrometer beyond one VIPA FSR a second dispersive element in a cross-orientation with the VIPA may be added after the VIPA to allow separation of these otherwise overlapping wavelengths. This second dispersive element could be a dispersing prism or a diffraction grating.
The overall spectral resolution of a VIPA-based spectrometer is defined not only by the spectral resolution of the VIPA, but also by the pixel size of a camera used to detect the dispersed light, the size of the pixel array of the camera, and the focal length of a lens that focuses the dispersed light upon the camera. Typically there is a trade-off between the spectral resolution and the measurement spectral window of the spectrometer, so that a spectrometer with a greater spectral resolution will likely have a narrower measurement spectral window.
Thus there is a need for an optical spectrometer that combines a high spectral resolution with a broader operating wavelength range.
SUMMARY OF THE INVENTIONAccordingly, the present disclosure relates to a spectrometer comprising: a dispersive system comprising two dispersive elements sequentially disposed to disperse input light in two dimensions to produce dispersed light, wherein one of the two dispersive elements has an FSR that is smaller than an operating wavelength range of the spectrometer; one or more detector arrays for detecting the dispersed light; and, two lenses for separately focusing the dispersed light upon the one or more detector arrays; wherein one of the two lenses has a focus distance that is at least 2 times greater than the other of the two lenses.
An aspect of the present disclosure relates to a spectrometer comprising: a dispersive system comprising two dispersive elements sequentially disposed to disperse input light in two orthogonal dimensions to produce dispersed light, wherein one of the two dispersive elements has an FSR that is smaller than an operating wavelength range of the spectrometer; two two-dimensional (2D) detector arrays for detecting the dispersed light; and, two lenses for separately focusing the dispersed light upon the two 2D detector arrays; wherein one of the two lenses has a focus length that is at least 2 times greater than the other of the two lenses. In at least some implementations, the dispersive system is configured to output the dispersed light comprising two light beams corresponding to two different dispersion orders of one of the two dispersive elements, wherein one of the two lenses is disposed in an optical path of one of the two light beams and the other of the two lenses is disposed in an optical of the other of the two light beams at locations wherein the two light beams are at least partially spatially separated, and the two 2D detector arrays are disposed in the focal planes of the two lenses to separately receive the two light beams focused by the respective lenses.
An aspect of the present disclosure provides a spectrometer comprising: first and second 2D detector arrays; a dispersive system comprising a VIPA and a second dispersive element sequentially disposed in an optical path of input light for dispersing thereof in two dimensions, wherein the VIPA is characterized by an FSR that is smaller than an operating wavelength range of the spectrometer, and wherein the dispersive system is configured to output dispersed light comprising first and second light beams corresponding to two different dispersion orders of one of the VIPA and the second dispersive element; and, first and second lenses disposed for separately focusing the first and second light beams upon the first and second detector arrays, respectively; wherein the second lens has a focus distance that is at least 2 times greater than the first lens, so as to provide a greater wavelength resolution in a narrower wavelength band than the first lens.
An aspect of the present disclosure provides a method for analyzing an optical spectrum of input light using a first dispersive element having an FSR that is smaller than a target operating wavelength range of the input light. The method comprises: a) sending the input light onto the first dispersive element that is disposed for dispersing light in a first dispersion direction in accordance with a wavelength content thereof; b) sending the input light onto the second dispersive element that is disposed for dispersing light in wavelength along a second dispersion direction that is generally orthogonal to the first dispersion direction; c) focusing a first portion of the input light dispersed by the first and second dispersive elements in a first dispersion order of one of the first and second dispersive elements upon a first 2D detector array using a first lens; and, focusing a second portion of the input light dispersed by the first and second dispersive elements in a second dispersion order of the one of the first and second dispersive elements upon a second 2D detector array using a second lens that has a greater focus length than the first lens, so as to provide a greater wavelength resolution from the second detector array and a greater wavelength range from the first detector array.
Another aspect of the present disclosure relates to a spectrometer comprising: a dispersive system comprising at least one dispersive element configured to disperse input light to produce dispersed light of two dispersion orders; first and second detector arrays; and first and second lenses disposed to intersect different portion of the dispersed light, wherein the first detector array is disposed in the focal plane of the first lens to detect an illumination pattern of a first of the two dispersion orders, and the second detector array is disposed in the focal plane of the second lens to detect an illumination pattern of the second of the two dispersion orders.
The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, in which like elements are indicated with like reference numerals, which are not to scale, and wherein:
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, sub-systems, optical and circuit components, mechanical elements, assemblies, or techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and tools are omitted so as not to obscure the description of the present invention.
Note that as used herein, the terms “first”, “second” and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element or operation from another unless explicitly stated.
With reference to
Continuing to refer to
Referring to
The spectral resolution and the overall bandwidth of the spectrometer of
P1=D1·Lf·δλ≅2a . . . 3a. (1)
A similar relationship may hold for the grating dispersion direction, where the product P2 of the grating dispersion D2 times the lens focal length Lf times the VIPA FSR Δ could preferably be select to be of the size of 2 to 10 pixels, or more preferably 4 to 8 pixels:
P2=D2·Lf·Δ≅4a . . . 8a. (2)
The total operating wavelength range Δλ, of the spectrometer under such conditions depends on the number of pixels in the grating dispersion direction Np2, and may be given by the expression
Δλ=(Np2·a)/(D2·Lf)=(Np2·Δ)/(P2/a), (3)
where Np2·a=Lz is the size of the detector array in the grating dispersion direction. By way of example, if the size of one pixel a≤2 μm, P2≅8a may be selected in order to fully resolve the bright stripes 27 with minimal cross talk between adjacent stripes. The total operating wavelength range Δλ, of the spectrometer under such conditions may then be given by the number of pixels in the grating dispersion direction Np2 times the FSR of the VIPA Δ divided by 8, Δλ, =(Np2·Δ)/8. Note that equations (1)-(3) assume square or circular pixels for clarity with the same size in the z and y directions; one skilled in the art will however have no difficulty to adopt these equations to elongated pixels.
Thus, for a given combination of VIPA 6, grating 7, pixel size of camera 9, and array size of camera 9, the focal length Lf of the output lens determines both the available resolution and the total wavelength range which can be evaluated with a single exposure. The greater the focal lens, the better is the wavelength resolution of the spectrometer, as δλ˜1/Lf from equation (1). The lower the focal length, the larger the wavelength which can be covered, as evident from equation (3). Lens 8 with a focal length that is too short will limit the performance of the spectrometer either by causing the stripes to overlap, which hinders the unwrapping of the spectrum, or by causing resolvable spectral features to overlap on a single pixel in the VIPA dispersion direction. Thus, for a spectrometer with a given size of the output camera and its pixels, the selection of the output lens 8 involves a trade-off between the spectral resolution and the operating wavelength range.
Turning now to
The input lenses 13 and 14, VIPA 16, and grating 17 may operate generally as described hereinabove with reference to
Continuing to refer to
The operating wavelength range is often expressed as a multiple of the spectrometer's finest wavelength resolution. In the case of a resolution of 1 pm, a wavelength range of 10 nm would equate to 10 000 times coverage. Preferably, with the high resolution setting of the zoom lens, the wavelength coverage may be between 5000 and 15000 times the maximum resolution. A more general wavelength range of 3000 to 30000 times resolution may also be realized, and possibly a range as small as 1000 times or as large as 60000 times the maximum resolution. In some embodiments when the zoom lens is set for low resolution, the observable resolution may be approximately 20-30 times lower than in the high resolution setting, but the wavelength range would proportionally increase to 75000-125000 times the instruments highest resolution. More generally a wavelength range between 50000 and 150000 times the maximum resolution would be obtained with the low resolution setting. In the low resolution setting, the wavelength range could be as little as 30000 times the instrument's highest resolution or as much as 200000 times the instrument's highest resolution. All of these values will scale with the resolution achievable by the VIPA 16 provided the diffraction grating 17, and imaging optics are scaled appropriately.
Continuing to refer to
Both the first and second embodiments of the spectrometer of
Referring now back to
Referring to
Referring to
In the embodiment of
Turning now to
Thus an aspect of the present disclosure provides a method for analyzing an optical spectrum of input light will now be described with reference to
At step 81, the input light is dispersed with a first dispersive element in a first dispersion direction in accordance with a wavelength content thereof; in at least some embodiments the first dispersive element, such as a VIPA 16, may have an FSR that is smaller than a target operating wavelength range; this method. At step 82, the input light dispersed by the first dispersive element is sent onto a second dispersive element that is disposed for dispersing light in wavelength along a second dispersion direction that may be generally orthogonal to the first dispersion direction. At step 83, a first portion of the input light dispersed by the first and second dispersive elements in a first dispersion order of one of the first and second dispersive elements is focused upon a first two-dimensional (2D) detector array using a first lens. At step 84, which may be concurrent with step 83, a second portion of the input light dispersed by the first and second dispersive elements in a second diffraction order of the one of the first and second dispersive elements is focused upon a second 2D detector array using a second lens that has a greater focus length than the first lens, so as to provide a greater wavelength resolution in a relatively smaller spectral window from the second detector array, and a greater wavelength range with a lower spectral resolution from the first detector array.
With reference to
In various embodiments described hereinabove a recalibration of the spectrometer after changing the zoom lens setting, or the grating rotation, may be performed by measuring a known spectrum, for example from a mercury lamp, or some other lamp with a suitable spectrum. A motorized rotation stage, combined with automatic identification of spectral features from the calibration source, would enable rapid adjustment of center wavelength of the spectrometer.
Table 1 provides exemplary ranges of several parameters that may be relevant to one or more of the described embodiments, which are however not limited by these ranges.
It will be appreciated that the term ‘lens’ as used herein encompasses single lenses as well as lens systems composed of two or more lens elements.
The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. For example, each of the example embodiments described hereinabove may utilize other optical elements; in other embodiments functions of two or more optical elements may be combined in a single component. Furthermore, although the example embodiments are described hereinabove with reference to a VIPA as a first dispersive element, other embodiments may utilize other dispersive elements in its place, such as but not limited to a regular Fabry-Perot etalon, an echelle grating. Similarly, embodiments may be envisioned where the second dispersive element 17 is not a diffraction grating but for example a prism, or any other suitable dispersive element, wherein or the second dispersive element 17 is absent altogether from the spectrometer; in the latter case the detector arrays 21, 22 may be one-dimensional, such as e.g. a linear array of suitable photodiodes. Furthermore, elements or features that are described hereinabove with reference to a particular example embodiment may also relate to other described embodiments. All such and other variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.
Claims
1. A spectrometer comprising:
- a dispersive system comprising two dispersive elements sequentially disposed to disperse input light in two dimensions to produce dispersed light, wherein one of the two dispersive elements has a free spectral range (FSR) that is smaller than an operating wavelength range of the spectrometer;
- one or more detector arrays for detecting the dispersed light; and,
- two lenses for separately focusing the dispersed light upon the one or more detector arrays;
- wherein one of the two lenses has a focus distance that is at least 2 times greater than the other of the two lenses.
2. The spectrometer of claim 1 wherein one of the two dispersive elements has an angular dispersion that is at least 10 times greater than the other of the two dispersive elements.
3. The spectrometer of claim 2 wherein the one of the two dispersive elements comprises at least one of a virtually imaged phase array (VIPA), an echelle grating, or a Fabry-Perot etalon, and the other of the two dispersive elements comprises at least one of a prism or a diffraction grating.
4. The spectrometer of claim 2 wherein:
- the dispersive system is configured to output the dispersed light comprising two light beams corresponding to two different dispersion orders of one of the two dispersive elements,
- wherein one of the two lenses is disposed in an optical path of one of the two light beams and the other of the two lenses is disposed in an optical path of the other of the two light beams at locations wherein the two light beams are at least partially spatially separated, and
- the one or more detector arrays comprise two detector arrays disposed to separately receive the two light beams focused by the two lenses.
5. The spectrometer of claim 4 wherein the one of the two lenses with the longer focal length is disposed in the optical path of one of the two light beams that corresponds to a greater dispersion order than the other of the two light beams.
6. The spectrometer of claim 4 comprising a reflector disposed in the optical path of one of the two light beams and out of the optical path of the other of the two light beams so as to further spatially separate the two light beams prior to the two lenses.
7. The spectrometer of claim 1 including a beam splitter disposed in an optical path of the dispersed light to split the dispersed light into the first and second light beams, wherein one of the two lenses is disposed in an optical path of one of the two light beams and the other of the two lenses is disposed in an optical of the other of the two light beams, and wherein the one or more detector arrays comprise two detector arrays disposed to separately receive the two light beams focused by the two lenses.
8. The spectrometer of claim 1 including a mount movably holding the two lenses for interchangeably inserting either one of the two lenses in an optical path of the dispersed light.
9. The spectrometer of claim 3 wherein the one of the two dispersive elements comprises the VIPA, comprising an input port for receiving the input light, and one or more input lenses disposed to bring the input light to a line focus at the VIPA.
10. The spectrometer of claim 9, wherein the other of the two dispersive elements comprises the diffraction grating disposed optically following the VIPA to disperse the input light in a direction that is generally orthogonal to a VIPA dispersion direction.
11. A spectrometer comprising:
- first and second two-dimensional detector arrays;
- a dispersive system comprising a virtual image phase array (VIPA and a second dispersive element sequentially disposed in an optical path of input light for dispersing thereof in two dimensions, wherein the VIPA is characterized by a free spectral range (FSR) that is smaller than an operating wavelength range of the spectrometer, and wherein the dispersive system is configured to output dispersed light comprising first and second light beams corresponding to two different dispersion orders of one of the VIPA and the second dispersive element; and,
- first and second lenses disposed for separately focusing the first and second light beams upon the first and second detector arrays, respectively;
- wherein the second lens has a focus distance that is at least 2 times greater than the first lens, so at to provide a greater wavelength resolution in a narrower wavelength band than the first lens.
12. The spectrometer of claim 11 comprising an input port for receiving the input light, and one or more input lenses disposed to bring the input light to a line focus at the VIPA.
13. The spectrometer of claim 12, wherein the second dispersive element comprises at least one of: a dispersion grating and a prism that is oriented with a dispersion direction that is generally orthogonal to a VIPA dispersion direction.
14. The spectrometer of claim 13 wherein the second dispersive element optically follows the VIPA.
15. The spectrometer of claim 11 including a reflector disposed in an optical path of one of the first and second light beam so as to further separate the first and second light beams.
16. The spectrometer of claim 11 including a reflector disposed in an optical path of one of the first and second light beams so as to further separate said light beams at the first and second lenses.
17. The spectrometer of claim 11 wherein one of the first and second lenses is disposed to receive a portion of the input light that is dispersed by the VIPA in a first dispersion order, and the other of the first and second lenses is disposed to receive light that is dispersed by the VIPA in a second dispersion order.
18. The spectrometer of claim 11 wherein one of the first and second lenses is disposed to receive a portion of the input light that is dispersed by the second dispersive element in a first dispersion order, and the other of the first and second lenses is disposed to receive light that is dispersed by the second dispersive element in a second dispersion order.
19. A method for analyzing an optical spectrum of input light using a first dispersive element having a free spectral range (FSR) that is smaller than a target operating wavelength range, the method comprising:
- sending the input light onto the first dispersive element that is disposed for dispersing light in a first dispersion direction in accordance with a wavelength content thereof;
- sending the input light onto the second dispersive element that is disposed for dispersing light in wavelength along a second dispersion direction that is generally orthogonal to the first dispersion direction;
- focusing a first portion of the input light dispersed by the first and second dispersive elements in a first dispersion order of one of the first and second dispersive elements upon a first two-dimensional (2D) detector array using a first lens; and,
- focusing a second portion of the input light dispersed by the first and second dispersive elements in a second diffraction order of the one of the first and second dispersive elements upon a second 2D detector array using a second lens that has a greater focus length than the first lens, so as to provide a greater wavelength resolution from the second detector array and a greater wavelength range from the first detector array.
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Type: Grant
Filed: Jan 17, 2018
Date of Patent: Dec 3, 2019
Patent Publication Number: 20180202862
Assignee: LightMachinery Inc. (Ottawa, Ontario)
Inventors: Hubert Jean-Ruel (Ottawa), John Reid (Stittsville), John H. Hunter (Almonte), Jesse Dean (Ottawa), Edward S. Williams (Kanata), Ian J. Miller (Ottawa)
Primary Examiner: Maurice C Smith
Application Number: 15/873,547
International Classification: G01J 3/26 (20060101); G01J 3/02 (20060101); G01J 3/28 (20060101); G01N 21/25 (20060101); G01J 3/04 (20060101); G02B 26/08 (20060101); G01J 3/18 (20060101); G02B 27/10 (20060101);